CXX-1109 Add collMod integration test for changing TTL expiry time
CXX-1109 Add collMod integration test for changing TTL expiry time

The existing collMod integration test changes the validation document
on a collection, which uses a feature unsupported by server versions
older than 3.2.  This commit adds a new collMod test, which will be
able to give us coverage for this command against old server versions.

acmorrow/mongo-cxx-driver,mongodb/mongo-cxx-driver,xdg/mongo-cxx-driver,acmorrow/mongo-cxx-driver,acmorrow/mongo-cxx-driver,mongodb/mongo-cxx-driver,xdg/mongo-cxx-driver,xdg/mongo-cxx-driver,mongodb/mongo-cxx-driver,acmorrow/mongo-cxx-driver,xdg/mongo-cxx-driver,mongodb/mongo-cxx-driver